A common type of gripping device is the "scissor" type gripper, which is of simple construction and operation but tends to push the workpiece away as it closes. The standard parallel jaw type gripper which is widely used has been made in a variety of forms but characteristically, this type of gripping device maintains only a single line or point contact with a cylindrical workpiece. If the workpiece is very long or heavy, such a precarious grip makes it difficult or impossible to control and maneuver. In addition, there are numerous other grippers which are contoured to exactly fit the workpiece but have the disadvantage that their suitability is limited to handling only one workpiece diameter or configuration.
U.S. Pat. No. 4,572,564 discloses a robotic adaptive gripping device operationally similar to a workbench vise with parallel clamping surfaces which adjust to fit the shape of the object being grasped and U.S. Pat. No. 4,623,183 discloses a robot hand gripping device utilizing six motors for operation and three concentric shafts which can be rotated independently of each other for orienting three gripper fingers. Each of these devices has little moment-carrying capability. U.S Pat. No. 4,403,801 discloses a clamp assembly having opposed pressurizable gripping cylinders and padded surfaces which enables the clamp to conform to objects of more than one shape. However, it is designed for handling relatively short workpieces and accordingly lacks moment-carrying capability for handling long workpieces. U.S. Pat. No. 4,367,893 discloses a gripping device with three gripping arms driven simultaneously by a single motor to close around a cylindrical object while effecting the centering of the workpiece. The device has a limited capture envelope for shafts restricted by a U-shaped shaft accommodating opening and utilizes three points of contact for grasping rather than multiple lines of contact. U.S. Pat. No. 4,368,913 discloses a parallel jaw type gripping device which employs parallel linkage systems for jaw operation wherein the gripping members are contoured to fit the workpiece and close in a straight line movement. The device has minimal moment-carrying capability.